The measurement of blood pressure is one of the oldest arts in the field of medicine, dating back some 350 years. Today, the measurement of blood pressure can be done both indirectly, by use of a sphygmomanometer, or directly by means of a device which is in direct contact with the circulatory system. The former is the most common in routine physical examinations, and the latter the most commonly used blood pressure device during specialized surgical procedures. The instant invention relates to a direct method of measuring a patient's blood pressure using a fiber optic intravascular transducer during surgery or in a routine catheterization test.
Before the introduction of fiber optic technology in the 1960's and 1970's, there were two principal types of devices developed for direct blood pressure measurements. These types of devices have been in use in hospitals since then without any major improvements in the art of direct blood pressure measurement.
The first type of device is a so called fluid-filled catheter manometer. It is composed of two basic parts; a hollow catheter and a pressure sensor. According to this type of device, the catheter is filled with a saline solution and then introduced into a blood vessel while the pressure sensor is placed outside the patient's body. The fluid filled catheter provides a hydraulic connection between the source of the vascular blood pressure and the sensor element.
This hydraulic pressure transmission system with its external sensor is relatively simple, durable, flexible and inexpensive. However, inherent in this device are a number of problems in the measurement of vascular blood pressure.
First, the nature of the saline solution and the flexible catheter wall material yield very poor dynamic performance of the system. A typical resonant frequency of 15-20 Hz, far below the minimum required 40-50 Hz, plus a non-optimal damping factor, cause distortion in the measured blood pressure waveforms. Second, the liquid-coupling system needs to be flushed at regular intervals to avoid blood clotting at the catheter tip. Thus, long-term measurements cannot be made with this device. Third, any bubbles existing in the fluid-filled system will not only degrade the measurements of blood pressure, but will also remain lethal threats to the patient's life. Finally, artifacts are often produced due to body motion, the relative large mass of the fluid column, and relative long transmission distance for blood pressure.
Many of the above problems associated with a fluid-filled catheter are eliminated by use of the second type of device which makes use of a semiconductor pressure sensor at the catheter tip. The prime advantage of the semiconductor catheter-tip blood pressure transducer is that the measurement of vascular pressure is made at the same place where it occurs rather than relying on the fluid coupling system to transmit pressure to an external sensor.
Although it produces artifact-free performance, the semiconductor catheter-tip pressure transducer continues to have other unsolved problems. Because of the electrical connection between the patient's body and the electronic device, a risk is imposed to the patient's health by possible excessive electrical current leakage which can disturb the normal electrophysiology of the heart. This can lead to the onset of cardiac arrhythmia and electric shock. This risk of cardiac arrhythmia and electrical shock is one of the medical profession's major concerns with the semiconductor type of catheter-tip transducer. In addition, the complicated construction causes this type of transducer to be a high cost product, which renders it impractical as a disposable device. Moreover, the multiple use of this type of a transducer not only consumes a great deal of labor time to maintain the transducer but also introduces possible cross contamination problems.
The development of fiber optic technology has solved many of the problems that existed in prior direct blood pressure measurement devices. A device employing fiber optic technology is capable of performing as well as a semiconductor catheter tip pressure sensor, and at the same time eliminates the two major problems associated with the semiconductor device, electrical leakage and cross contamination.
The work on fiber optic catheter-tip blood pressure transducers started in the 1960's. In a typical early model of a fiber optic catheter tip pressure transducer, a pressure sensitive membrane is mounted at the catheter tip where an end opening is made to allow the direct contact of the membrane to the measured blood pressure. Right behind the membrane is an optical fiber bundle which stays inside the catheter. At the proximal end, the fiber bundle bifurcates into two legs connected to a light source and a light detector, respectively. Light from the light source is transmitted through those fibers associated with that leg of the bundle and then reaches the pressure sensitive membrane with light reflecting properties. Part of the reflected light is collected by those fibers in the bundle which go to the light detector and is transmitted back to the detector. At the distal end, all of these fibers are made part of a single bundle. When a pressure is applied and coupled through the end opening at the catheter tip to the membrane, the intensity of the reflected light will be altered by the displacement of the membrane. Thus, the signal generated by the light detector will be changed in direct proportion to the applied pressure. The pressure signal is then processed and displayed (or recorded).
There are at least two problems associated with this type of fiber optic transducer, and both are related to the end opening at the catheter tip. The placement of the pressure detection means at the distal end opening subjects the pressure measurement to a source of error. When introduced into the blood stream, the catheter is parallel to the direction of the blood flow. The end opening of the catheter faces either the upstream or down stream flow of blood. In this arrangement, based on Bernoulli's law, the total measured pressure will be the static blood pressure plus or minus the kinetic energy pressure. The static blood pressure is the desired parameter and the kinetic energy pressure is the introduced error. This error may be as large as 10% of measured pressure when the patient is at rest or 50% when the patient is in an active state. In addition to the measurement error, the edges at the catheter tip necessary with this type of configuration will compound thrombus formation.
To overcome the above problems associated with the end opening at the catheter tip, two types of fiber optic catheter pressure sensors having a side opening at the catheter tip are reported in a 1978 article entitled "The Development of Fiber Optic Catheter Tip Pressure Transducer", Journal of Medical Engineering and Technology, Vol. 2, No. 5, by H. Matsumoto & M. Saegusa, and disclosed in 1987 U.S. Pat. No. 4,691,708 entitled "Optical Pressure Sensor for Measuring Blood Pressure" of J. Kane.
In Matsumoto's disclosure, a membrane to cover the side opening at the catheter tip and a cantilever are responsive to applied pressure. The displacement of the whole structure causes movement of a mirror which is mounted at the end of the cantilever. The position of the mirror will alter the active reflection surface available for the inlet light transmitted by fiber optic means extending the length of the catheter so that the produced pressure signal at the light detector is in direct proportion to the static blood pressure. However, it is extremely difficult to precisely determine the initial position of the micro-mirror relative to the distal end of the optical fibers. A very slight misalignment between them will destroy the sensor's performance completely.
In Kane's patent, a pressure transducer is located at the distal end of the catheter and includes a fixed mirror which is spaced forward of the distal end of the single optical fiber. This mirror is fixed in position relative thereto, so as to receive and reflect light emitted from the distal end of the optical fiber and back into the distal end thereof. A side port is provided in the catheter housing adjacent to its distal end, which is sealed with a plastic membrane responsive to pressure acting transversely thereof. The plastic membrane is coupled to the distal end of the optical fiber through a wedge-shaped bias support. Under the influence of the applied pressure, the distal end of the fiber is displaced, thus changing the amount of reflected light.
There a number of problems with Kane's transducer. First, it ignores the initial distance and angle between the distal end of the optic fiber and the fixed mirror. Misplacement of these two parameters will sacrifice greatly the transducer's sensitivity, dynamic range and linearity. The transducer structure provides no means to accurately and precisely preset and trim the initial distance and angle between the distal end of the optic fiber and the fixed mirror. This will cause extreme difficulty in maintaining reproducibility in the sensor's performance. In addition, both static and dynamic pressure impacts can deform its flexible structure to some extent because metal is not employed to increase the strength of the pressure sensor. This can change the above two parameters thus introducing significant artifacts into pressure measurements. Secondly, the plastic membrane used has poor frequency response and large hysteresis which can cause distortion in measured pressure waveforms. The pressure deformation of plastic material also produces significant baseline shift. Finally, this transducer needs a very expensive means to convert the one fiber used into the two legs connected to the light source and the light detector. Thus, Kane's design has not proven to be practically useful.